This invention relates to electret condenser acoustic transducers and more particularly to a method of providing supporting posts for spacing the diaphragm in a transducer with respect to a backplate which constitutes a fixed electrode.
In the manufacture of electret condenser transducers, e.g. microphones, various methods have been proposed for precisely maintaining the desired small separation between the diaphragm which constitutes a moving electrode and a backplate which constitutes a fixed electrode. In addition to various schemes for clamping the diaphragm at its edges, methods have been proposed for forming regular or irregular patterns of ribs, bumps or posts which separate the two elements and effectively divide the diaphragm area into a plurality of regions. Several such methods are disclosed, for example, in the P. V. Murphy reissue Pat. Re. 28,420. As the usual diaphragm material comprises a thin film which is under tension, the straightforward and therefore apparently universally employed method of generating the post pattern is to form it on the backplate while the diaphragm is made comprising the material which can be polarized as an electret.
In accordance with the present invention, however, it has been found that substantial unexpected advantages can be obtained if the post pattern is formed on the diaphragm, despite its thin film nature. A principal advantage is that the backplate, i.e. fixed electrode, may then be formed with a smooth, flat surface and the electret material can then be laid down on that surface. While most of the usual electret materials are plastics or resins, they do not have mechanical properties which are themselves ideal for use as a diaphragm. For example, Teflon, (polytetrafluoroethylene) one of the most stable and commonly used electrets, is not particularly tough or elastic so as to render it ideal for a highly compliant acoustically responsive diaphragm such as is desired in constructing a measurement grade condenser microphone.
In accordance with another aspect of the invention, it has been found that supporting posts appropriate for defining the spacing between the diaphragm and backplate in an electret transducer may be expeditiously formed on a plastic film suitable for use as a diaphragm by conventional photo-etching techniques.